Many years ago when I was a child, my grandfather explained to me why the roads in Galicia were so winding: They had been laid out following the pathways packhorses chose when crossing the mountains. Somehow the capability of many animals to find their way over apparently trackless forests has been far better that ours for years before the advent of cartography and navigation technologies becoming widely accessible.

Horses are not particularly famous for being intelligent animals, but they are supposed to be more intelligent than amoebas, aren’t they? Well, it depends: amoebas seem to be better civil engineers than horses. They are able to lay out very detailed plans for transportation networks in cities or even a whole country.

Slime mould is a broad term describing organisms that are neither animal, nor plant nor fungus^(*). Slime moulds start life as single, amoeba-like cells. Eventually they find mates and produce offspring that grow into so-called plasmodia —structures that are full of cell nuclei, but without any dividing cellular walls. Plasmodium of Physarum polycephalum is a unique living substrate which has proved to be efficient in solving labyrinthine mazes and other shortest path problems.

When P. polycephalum is foraging, it sends out protrusions of protoplasm that continue to branch out until it finds a food source, where it forms a blob around the nutrients. In this way, the plasmodium builds an efficient foraging and intra-cellular transportation network which somehow trades off cost, efficiency and resilience as it expands.

The problem P. polycephalum solves closely resembles the problem of building transportation networks across a country, so researchers have studied how close plasmodium of P. polycephalum approximates man-made motorway and rail networks in many different places^(1,2,3). And, as you can see the results are striking.

What’s even more surprising is that P. polycephalum is a completely brainless mould. How does it do it? Spatial memory enhances an organism’s ability to navigate in complex environments. Can you have a memory if you don’t have a brain?

Results from insect studies, for example ants leaving pheromone trails, challenge the assumption that navigation requires learning or a sophisticated spatial awareness. By depositing pheromone, individual ants externalize their memory of the route from the nest to a food source. Externalizing their memory frees foragers from having to store memories within themselves and the network of pheromone trails within the environment is the externalized collective memory of the entire colony.

Slime moulds also use excreted chemicals as an externalized memory system⁽⁴⁾. The brainless P. polycephalum constructs a form of spatial memory by avoiding areas it has previously explored. This mechanism allows the slime mould to solve the U-shaped trap problem—a classic test of autonomous navigational ability commonly used in robotics.

The deposition and reaction to chemical markers in the environment (stigmergy) was most likely a functional precursor to internal memory, allowing biological systems with primitive information-processing systems to solve tasks requiring spatial memory. Reid and coworkers⁽⁴⁾ think that memory of multi-cellular organisms might have evolved by using external chemical trails in the environment before the development of internal memory systems.

So before you go to your local store to buy a larger hard disk to put a bit of order in your life, think twice. Maybe what appears to be a messy room is just your externalized memory!

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(*) Biologist Audrey Dussutour affectionately call it the blob (see figure)

(1) Atsushi Tero et. al. “Rules for Biologically Inspired Adaptive Network Design”, Science Vol. 327 no. 5964 pp. 439-442;  January 2010

(2) Andrew Adamatzkya & Ramon Alonso-Sanzb, “Rebuilding Iberian motorways with slime mould”, Biosystems
Volume 105-1, pp. 89–100; July 2011

(3) Andrew Adamatzky , Andrew Ilachinski, “Slime Mold Imitates the United States Interstate System”, Complex Systems, 21-1; 2012

(4) Chris R. Reid, Tanya Latty, Audrey Dussutour, and Madeleine Beekmana, “Slime mold uses an externalized spatial “memory” to navigate in complex environments”, PNAS vol. 109 no. 43; October 2012

Featured Image: “Messy Room”, Daniel Gafanhoto